1. Field of the Invention
The present invention relates to a power-factor control circuit in an AC-to-DC converter, and, more particularly, to a small-sized AC-to-DC converter which can be manufactured at low cost and maintains an output voltage thereof with high efficiency.
2. Description of the Related Art
AC-to-DC converters are used generally as switching regulators for audio system. FIG. 1 shows an example of the structure of a conventional AC-to-DC converter. A full-wave rectifier, comprising four diodes 72 to 75 in a power-factor control circuit 71 shown in FIG. 1, rectifies a voltage applied by an AC power source 70. A switching controller 80 applies a driving signal to a switching device 79, in order to control the xe2x80x9copenxe2x80x9d and xe2x80x9cclosedxe2x80x9d conditions of the switching device 79. In the case where the switching device 79 is closed, the electromagnetic energy is accumulated in an inductor 76. In this case, the current flowing to and through the inductor 76 increases in proportion to time. In the case where the switching device 79 is open, the electromagnetic energy is transmitted from the inductor 76 to a capacitor 81 through a diode 77. As a result, the capacitor 81 is charged. To prevent the switching device 79 from being destructed due to heat and prevent an increase in the loss of power consumption, an output current of the power-factor control circuit 71 is limited by a non-illustrative protecting circuit. Without the protecting circuit, the copper loss or iron loss in the inductor 76 increases. Otherwise, the core saturation may occur in the inductor 76.
In accordance with a switching operation of such a switching device 79, an output voltage (i.e. a voltage of the capacitor 81) of the power-factor control circuit 71 is retained at a constant voltage greater than the amplitude of the voltage supplied from the AC power source 70, regardless of the size of a voltage input from the AC power source 70 to the power-factor control circuit 71. For example, in the case where the AC power source 70 is 100 volt AC, the output voltage of the power-factor control circuit 71 is retained at 160V or 190V. In the case where the AC power source 70 is 240 volt AC, the output voltage of the power-factor control circuit 71 is retained at 400V.
Two switching devices 83 and 84 included in a DC-to-DC converter 82 connected to the capacitor 81 are complementary switching devices. In other words, when the switching device 83 is open, the switching device 84 is closed, and when the switching device 83 is closed, the switching device 84 is open. As a result of this switching operation of the switching devices 83 and 84, currents flow to the primary winding of the transformer 85 alternatively, and an AC voltage is generated by the secondary winding of the transformer 85. A rectification circuit comprising diodes 88 to 91 rectifies a voltage received from the transformer 85, to generate a DC voltage. The DC voltage output from the rectification circuit is applied to capacitors 92 and 93. The voltage at the capacitors 92 and 93 are retained at a constant level by a non-illustrative voltage controller. The voltage at the capacitors 92 and 93 are applied to loads 94 and 95, and resulting in that DC power is transmitted to the loads 94 and 95.
In the normal use of the audio system, when an audio system generates a big sound, the average power consumed by the loads 94 and 95 is in a range between {fraction (1/50)} and xe2x85x9 of the maximum power consumption. Even in the case where the audio system generates a rare extremely big noise, the average power consumed by the loads 94 and 95 is in a range between one sixteenth and half of the maximum power consumption. Though the average consumed power is very little, it is necessary to design the switching regulator not to be destructed even in the state where the maximum power is consumed. As a result, the power can not effectively be consumed in the normal use of the AC-to-DC converter, and the improvement of the power factor can not result in a desirable effect. In addition, the cost of each component included in the AC-to-DC converter increases.
In contrast to the above, a large amount of power is consumed in a moment, when sounds of musical instrument (e.g. sounds of piano, drum, etc.) which have low frequency components and have an attack sound segment having the large amplitude, or when a mixed sound of a high tone sound and a low tone sound is output. By the effect of the restriction in an output current of the power-factor control circuit 71, currents flowing to and through the loads 94 and 95 are restricted as well. Hence, in the case where the power consumed by the loads 94 and 95 exceeds its restriction, the voltage applied to the loads 94 and 95 suddenly drops. FIG. 2 shows the relationship between output power of a DC-to-DC converter 82 and an output voltage of a power-factor control circuit 71, in the AC-to-DC converter shown in FIG. 1. As shown in FIG. 2, when the output power of the DC-to-DC converter 82 is the maximum power consumption Pmax [W], the output voltage of the power-factor control circuit 71 suddenly drops. In such circumstances, the linearity of the audio output from the audio system will remarkably be deteriorated. In the power-factor control circuit 71 wherein the output current is restricted in association with the power consumed by the loads 94 and 95, the inductor 76, the switching device 79 and the diode 77 are large in size, and hence the volume of the power-factor control circuit 71 will be increased. For example, the volume of the power-factor control circuit 71, whose output current is limited in association with the maximum consumption power of the loads 94 and 95, increases two or sixteen times more than that in the general case where the output current is controlled by the protecting circuit. Additionally, there will be an increase in the component cost and the cost of manufacturing the AC-to-DC converter.
The present invention has been made in consideration of the above. It is accordingly an object of the present invention to provide an AC-to-DC converter which is small in size, can be manufactured at low cost, and operates with high efficiency.
Another object thereof is to provide an AC-to-DC converter, whose output-voltage variation can be restricted, in the case where the AC-to-DC converter outputs a large amount of power.
In order to attain the above objects, according to one aspect of the present invention, there is provided an AC-to-DC converter comprising:
a power-factor control circuit which is coupled to an AC power source;
a capacitor which has a first electrode and a second electrode, and whose charge/discharge operation is controlled by the power-factor control circuit; and
a converter which is coupled to the capacitor, and
wherein the power-factor control circuit includes
a rectifier which is coupled to the AC power source,
a detector which detects a peak output voltage of the rectifier,
a switching device which executes a switching operation for charging the capacitor, and
a switching controller which controls the switching operation of the switching device, based on a detected result of the detector.
In this structure, the power-factor control circuit may charge the capacitor in accordance with the switching operation of the switching device, such that a voltage in the capacitor will be equal to the peak output voltage detected by the detector or greater than a peak output voltage of the detector by a predetermined amount of voltage.
The power-factor control circuit may include:
an inductor which has a first terminal, coupled to a positive output terminal of the rectifier, and a second terminal; and
a first device which is conducted to the capacitor in order to supply the capacitor with a current, and which is coupled between the second terminal of the inductor and the first electrode of the capacitor.
In this structure,
the switching device may be coupled between the second terminal of the inductor and the second electrode of the capacitor.
The first device may be a diode having an anode coupled to the second terminal of the inductor and a cathode coupled to the first electrode of the capacitor.
The inductor may have a third terminal; and
the first device may be an n-channel type field effect transistor having a source coupled to the second terminal of the inductor, a drain coupled to the first electrode of the capacitor, and a gate coupled to the third terminal of the inductor.
Otherwise, the inductor may have a third terminal; and
the first device may be a p-channel type field effect transistor having a source coupled to the first electrode of the capacitor, a drain coupled to the third terminal of the inductor, and a gate coupled to the second terminal of the inductor.
The power-factor control circuit may include a second device coupled to the inductor and the first device in parallel thereto, in order to transmit a current from the rectifier to the capacitor in a case where a voltage of the capacitor drops.
The second device may be a diode having an anode coupled to a positive output terminal of the rectifier and a cathode coupled to the first electrode of the capacitor.
The second device may include:
a first diode coupled between one end of the AC power source and the first electrode of the capacitor; and
a second diode coupled between other end of the AC power source and the first electrode of the capacitor.
The power-factor control circuit may:
control a charge/discharge operation of the capacitor in accordance with the switching operation of the switching device, in a case where output power of the AC-to-DC converter is equal or less than predetermined reference power; and
control the charge/discharge operation of the capacitor by a current flowing to and through the second device, in a case where the output power of the AC-to-DC converter is equal to or greater than the reference power.
The reference power may be within a range between one sixteenth and half of maximum power consumption.
The AC-to-DC converter may include a voltage controller for controlling an output from the converter.
The detector may include
a semiconductor element which has an anode coupled to a positive output terminal of the rectifier, and a cathode; and
a capacitive element which is coupled between the cathode of the semiconductor element and a negative output terminal of the rectifier.